When the flash of fireflies breaks the darkness on summer evenings, you’re seeing male fireflies broadcasting their light for females waiting in the grass. If a female is interested, she will flash in response. Studies have shown that the females prefer males with longer flashes and faster rhythms; however, if that is the case, why haven’t males steadily evolved longer and faster flashes?

Researchers at Tufts University in Boston, led by Sara M. Lewis and William A. Woods Jr., recently studied the Photinus fireflies to find out whether there are any hidden costs for flashier males.

Image courtesy of Sara M. Lewis, Tufts University.

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Using open-flow respirometry, which measures how much carbon dioxide a firefly produces, the researchers found that, while flashing, the male firefly consumes only nominally more energy than while resting. Because energy expenditure isn’t the hidden cost, they then tested the potential predation costs of the flash signals. Photinus fireflies produce noxious chemicals that deter most predators but that attract larger predatory fireflies called Photuris.

The researchers found that the predatory fireflies were attracted more often to fake firefly flashes than to nonflashing identical controls and were significantly more attracted to faster flashes. Thus, Photinus fireflies that are too flashy are more likely to be eaten by the predatory fireflies. The work will be published in the November issue of The American Naturalist.

These findings show that an evolutionary balancing act occurs between attracting a mate and avoiding getting eaten, and Lewis says it is possible that these opposing forces could cause a shift that generates entirely new firefly species with distinctive flash codes.